Broad band toroid r.f. transformer



July 26, 1966 i '5. N. ARvoNlo BROAD BAND TOROID R.F. TRANSFORMER Filed June 50, 1964 `ATToRNEYs United States Patent O 3,263,191 BROAD BAND TOROID R.F. TRANSFORMER Edward N. Arvonio, Watson and Almshouse Roads,

' P.0. Box 193, Jamison, Pa.

Filed `lune 30, 1964, Ser. No. 379,424 Claims. (Cl. 333-24) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

The present invention relates to radio frequency transformers and more particularly to a broad band toroid radio frequency transformer.

In the field of radio frequency transformers it has been the general practice to employ iron cores either in a straight configuration or in a toroid configuration. These devices have the difficulty that they can be tuned only for one particular frequency. More often used in a medium powered radio frequency amplifier is the well known pi network which consists of a large inductance, a band switch and two large tuning capacitors. This type of network is very effective electrically. However, the size, weight and control circuits required for automatic tuning increase the complexity of design. The use of ferrite in a transformer has been previously suggested but transformers constructed from ferrite materials were easily saturated at lower power levels and provided only a narrow band width of linear frequency response.

The purpose of this invention is to provide a transformer of ferrite or other lighter weight material which gives high power with high efficiency and a broad band response.` To attain this the present invention uses a ferrite or other light weight core with a bifilar primary windingv in which the two windings of the primary have a capacitive interaction to prevent saturation and to providea broad band linear response. The invention also comprises a new method of winding such a transformer to pQduce the aforesaid results. Accordingly, it is an object of the present invention to provide a broad band radio frequency transformer of lightweight material.

Another .object of the invention is to provide a radio frequency transformer of toroid configuration with a bifilar primary winding.

A further object of the invention is the provision of a radio frequency transformer which provides an automatic impedance match over a wide band of radio frequencies.

Still another object of the invention is to provide a method of winding a transformer so as to provide a broad band frequency response with light weight at high power.

With these and other objects in view as will hereinafter more fully appear and which will be more particularly pointed out in the appended claims reference is now made to the following description taken in connection with the accompanying drawings in which:

FIG. 1 shows a plan view of a toroid transformer of a preferred embodiment of the invention.

FIG. 2 illustrates a sectional view taken along line 2--2 of FIG. 1.

FIG. 3 shows a toroid transformer according to another embodiment of the invention.

FIG. 4 shows a sectional view taken along line 4-4 of FIG. 3.

FIG. 5 shows a graph of output power versus frequency for a typical transformer according to the invention.

FIG. 6 shows a graph of input impedance versus frequency for a typical transformer according to the invention.

The embodiment of FIG. 1 shows a toroid core 11 which may be any of various cross sections, square, rectangular, round or oval, but preferably is round or oval. The com- ICC position will be of any lightweight core material such as ferrite, ceramic or phenolic resins or any other composition which has a low loss dielectric as to radio frequency. A particular set of core materials made by General Ceramic Division, Indiana General Corporation are designated Q-l, Q-Z, Q-3 and Q-4 and are particularly suitable for the core material of the present transformer. An enamel coated wire is wound around the toroid for a number of turns sufficient to go completely around the toroid, and constitutes the first winding 12 of the primary winding. After this winding is put on, it is covered with a polytetrauoroethylene (Teflon) tape 13 as shown in FIG. 2. This tape is very thin but has excellent insulation qualities. Other plastic tapes capable of withstanding voltages of up to 1,000 volts are also suitable. The second winding 14 of the primary is then wound around the Teflon tape in between the spaces of the first winding of the primary, as shown in FIG. 2. Because of the thinness of the tape the second winding lies essentially side by side with the first winding of the primary, and the two windings of the primary may be considered to be bifiliarly wound, in that they continue around the toroid core in parallel. of the primary will have substantially the same inductive area within their centers. These two windings of the primary will have a capacitive relationship due to the parallel winding around the core as well as the inductance resulting from the helical winding. Each point in the primary coil will be in capacitive relationship with a point one half of the coil away from it. By means of adjustment of these two primary windings the core may be adjusted to resonate at the center of the radio frequency range desired. For example, if a frequency range of 3 megacycles to 4.5 megacycles as shown in FIG. 5 is desired the primary windings would be adjusted to resonate at approximately 3.8 megacycles. The secondary winding 15 is wound on top of the two windings of the primary and is adjusted to provide impedance match as will be described subsequently. The secondary is connected between an antenna 16 or other output load and ground.

FIG. 3 shows an alternate embodiment of the invention. A core similar to that of FIG. l is wound with a double strand wire 21. This wire 21 is the type of wire commonly known as a bifilar wind has two wires 22 and 23 within an insulation core. When this is used it is wound around the core 11 for the number of turns desired and then, as shown in FIG. 3, one end of 22 is linked with the end of 23 of the opposite end of the -biilar wind so that 22 becomes the first winding of the primary and 23 becomes the second winding in series with 22. This embodiment is not -as desirable as that of FIG. l for a number of reasons. One, it is impossible to adjust the turns of the first and second windings of the primary to resonate the coil to the particular frequency and therefore Ian added capacitance must :be used. Secondly, the voltages between the first and second windings of the primary can Ibe very high, as high as 1,000 volts or more. If the choice of composition of the biflar wind material is not very careful this voltage will be much too much and the insulation between the two windings will be broken down quite easily. One therefore must choose expensive insulation It also is of note that each of the two windings starts with the secondary wound near the grounded end of the primary. If a lower impedance is desired, the secondary is moved closer to the grounded end. If a higher impedance is desired the secondary is moved around the coil away from the grounded end. The number of respective turns in the primary and secondary will have already been chosen from the point of view of impedanoe and voltage ratios. It will be found that when the coil is Ibalanced for the center frequency it will also be balanced throughout the usable range of the coil which may be -expressed either as 25% plus or minus of the center frequency or 75% of the lowest frequency.

The broad band linear response of the transformer of the present invention stems from the fact that there is not only distributed inductance in the coil of the transformer but distributed capacitance as well between the first and -second windings of the primary. Due to this distributed inductance and capacitance the device appears as a transmission line and by proper selection of the inductance and capacitance it may be made to look like an electrical quarter w-ave transformer. yIt is found, contrary to the expectations in the art, that the core material in the present transformer does not saturate at high power levels even with materials such as ceramic and phenolic resins. Moreover, it is discovered that it retains these characteristics over a broad band of response. FIG. 5 shows a typical output power response for a transformer according to the present invention tuned from 3 megacycles to 4.5 megacycles. The band width shown in the figure is measured to the 1 db power level. If one goes down to the half power point, which is the 3 db level, the Iband width will be even wider. However, in general it is better to use the transformer only within the 1 db band width. It will he noted also that the efficiency of the transformer is very high, giving an output power of up to 400 watts p.e.p. for an input power of 7 O0 watts. This is an efficiency of over 55%. The input impedance provided by the transformer for a known antenna resistance is even steadier over the band width, as shown in FIG. 6. This substantially constant input impedance enables the preceding amplifier to be designed for maximum efficiency wit-h this load impedance. This is a vital necessity in producting high power amplifiers for transmitters. The input to output impedance ratio, or the plate to antenna impedance ratio, can be as high as 8O to l or higher. Typically the antenna will be approximately 50 ohms and the input resistance will be anywhere from 50() to 7,000 ohms.

In designing the band width of the transformers it is generally advisable to avoid a transformer that extends for more than one complete octave. The reason is that down at the lowest frequency of the band width the second harmonic of this frequency will be within the band path and will therefore be transmitted without attenuation. With the pass band actually selected in a constructed transformer there was an attenuation of the second harmonic of the input of 29 db. The second harmonic may be reduced by as much as 60 db with proper construction. However, if the input signal is completely linear with no harmonics it will be possible to cover a range of from 2 megacycles to 30 megacycles with a single transformer using solid state devices. With harmonics present in the input signal it is advisable to have about eight transformers to cover this range, each of the transformers operating over not more than three quarters ofone octave in this frequency range.

The transformer of the present invention substitutes for the pi network previously utilized, resulting in a great decrease of size, weight and control circuits as well as the elimination of the necessity for retuning the circuit for each frequency.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. A broad band radio frequency transformer for operation at a predetermined frequency range and at high power levels and high input to output impedance ratio comprising:

a toroid core;

a first winding wound on said core;

a second winding electrically in series with said first winding and wound directly on said core in close parallel relationship with said first winding, said first and second windings being adjusted with respect to each other around said core to resonate at the center of .said frequency range and comprising the primary of said transformer;

the inductance and capacitance of said first and second windings Ibeing selected to appear as .an electrical quarter wave transformer at said frequency range; and

a third winding wound over said first and second windings and comprisingv the secondary of said trans former.

2. A broad iband transformer as recited in claim 1 wherein said first and 4second windings of said primary have substantially the same inductive coil area.

3. A broad band transformer as recited in claim 2 wherein the first winding is laid on the core; a thin layer of polymer tape is laid over the first winding and lthe second winding is laid over the tape in the spaces on the core between the coils of said first winding.

4. A broad band transformer as recited in claim 3y wherein the second winding has the same number of turns as the first winding, and each point in the second winding is opposite a point in the first winding one-half of the primary coil length from it.

5. A broad 1band transformer as recitedv in claim, 2,

wherein said first and second windings comprise a double wire in a single insulated casing wound around the core together with one end of one of the wires linked to theA opposite end of the other wire so as to connect thetwo coils in series.

References Cited by the Examiner UNITED STATES PATENTS 2,195,233 3/1940 Boyer 3361-209 X 2,488,325 ll/l949 Peek 336--69 X FOREIGN PATENTS 918,978 2/1963 Great Britain.

LARAMIE E. ASKIN, Primary Examiner.

ROBERT K. SCHAEFER, Examiner.

r D. I. BADER, Assistant Examiner'. 

1. A BROAD BAND RATIO FREQUENCY TRANSFORMER FOR OPERATION AT A PREDETERMINED FREQUENCY RANGE AND AT HIGH POWER LEVELS AND HIGH INPUT TO OUTPUT IMPEDANCE RATIOS COMPRISING: A TOROID CORE; A FIRST WINDING WOUND ON SAID CORE; A SECOND WINDING ELECTRICALLY IN SERIES WITH SAID FIRST WINDING AND WOUND DIRECTLY ON SAID CORE IN CLOSE PARALLEL RELATIONSHIP WITH SAID FIRST WINDING, SAID FIRST AND SECOND WINDINGS BEING ADJUSTED WITH RESPECT TO EACH OTHER AROUND SAID CORE TO RESONATE AT THE CENTER OF SAID FREQUENCY RANGE AND COMPRISING THE PRIMARY OF SAID TRANSFORMER; THE INDUCTANCE AND CAPACITANCE OF SAID FIRST AND SECOND WINDINGS BEING SELECTED TO APPEAR AS AN ELECTRICAL QUARTER WAVE TRANSFORMER AT SAID FREQUENCY RANGE; AND A THIRD WINDING WOUND OVER SAID FIRST AND SECOND WINDINGS AND COMPRISING THE SECONDARY OF SAID TRANSFORMER. 